Texturing various questions questions - Cycles
1) The term you are thinking of is "texture seam". Generally, if you have one flat side of an object, you would try to unwrap and texture it to avoid seams. it might be that the texture you are using isn't suited to what you have. 

2) I quite favor using micro-displacement for close-up images that require a lot of detail. However, if you are not going to be quite close to your object, it is possibly better for you to use normal/bump mapping. It is far less computer intensive than displacement/subdivision.

3) Yes, you can reuse assets and textures. Taking your wooden beam example, you might find that creating only three different beams, and using these in your scene gives enough variation. There is a trade-off between saving time/effort and avoiding repetitive items and textures.